(1) Field of the Invention
The present invention relates to a method and to a device for stopping a turboshaft engine in nominal operation.
(2) Description of Related Art
A vehicle, and in particular an aircraft, may be provided with a plurality of engines.
Certain rotorcraft are thus provided with a plurality of engines for driving at least one rotor providing lift and possibly also propulsion. For example, the Super-Frelon helicopter possesses three turboshaft engines.
Conventionally, a turboshaft engine comprises a gas generator having at least one compressor, a combustion chamber, and a high pressure expansion assembly connected to the compressor.
The compressor may have a plurality of compression stages that may be axial and/or centrifugal. Likewise, the expansion assembly may comprise a plurality of expansion turbines.
In addition, the engine has at least one low pressure working turbine that may be free or else linked to the gas generator.
The operation of the engine is controlled by a management system. For example, one known management system is a full authority digital engine control (FADEC) system. A management system usually comprises a computer connected to various sensors and to a fuel metering unit. The fuel metering unit then enables the management system to control the rate at which fuel is fed to the engine.
The management system is conventionally connected to a selector that is referred to for convenience as the “control selector”. For example, the control selector may have three positions.
When the control selector is in a first position, e.g. referred to as its “STOP position”, the engine is stopped.
When the control selector is in a second position, e.g. a “FLIGHT position”, the engine is in operation.
The term “nominal operation” is used below to cover operation of the engine while the control selector is in the FLIGHT position. The engine can then operate while running at a variety of distinct operating ratings.
Finally, the control selector has an intermediate position, e.g. an “IDLE position”. When the control selector is in this IDLE position, the engine operates at an idling speed. Idling is used on the ground or in flight to reduce the power developed by the engine.
In order to stop an engine in nominal operation, a person may move the control selector from the FLIGHT position to the STOP position.
When the control selector is put into the STOP position, the management system closes the fuel metering unit to stop feeding fuel to the engine. The engine is then stopped quickly. The power developed by the engine decreases at a steep gradient, of the order of 20 percent of the current power per second.
The loss of power resulting from stopping an engine in flight gives rise to jolts, in particular causing the aircraft to perform yaw movements. These jolts are felt by the crew and the passengers of the aircraft. Such jolts are necessarily unwelcome, at least from a sensory point of view.
On the ground, the situation is different.
Firstly, the power developed by the engine while on the ground is generally low during nominal operation. The drop in power then has limited impact on the aircraft.
Secondly, any jolt tending to cause the aircraft to perform a yaw movement is limited by friction between the landing gear of the aircraft and the ground.
Furthermore, directly stopping the engine as required by moving the control selector from the FLIGHT position to the STOP position can, in the long run, lead to fuel coking in the engine. This coking phenomenon relates to the fuel being transformed into a solid that can lead to malfunctioning of the engine. In the long run, and by way of example, the solidified fuel may plug injectors, prevent the engine from starting, lead to a loss of power from the engine, or indeed cause the engine to stop when no stop command has been given.
In order to avoid this coking phenomenon, a pilot may begin by moving the control selector to the IDLE position and keep the engine in the IDLE position for a certain length of time, prior to putting the control selector into the STOP position. Spending time idling seeks to stabilize the temperature in the combustion chamber of the engine at a low temperature prior to complete shutting down of the engine.
Document U.S. Pat. No. 4,738,098 gives teaching of this type.
When performed in flight, such a procedure considerably increases the workload on a pilot. Furthermore, passing to idling speed can also give rise to disagreeable yaw movements since the decrease in power from the engine is sudden, just as it is for a stop required by using the STOP position of the selector. Furthermore, the procedure may be accompanied by the need to reduce the collective pitch of the blades of a rotor on a rotorcraft. Such a step then makes the procedure more complicated.
In the technological background, Document U.S. Pat. No. 4,479,619 proposes a power transmission system for three-engined helicopters.
Document U.S. Pat. No. 3,963,372 proposes a solution for managing power and for controlling the engines in three-engined helicopters. Document WO 2012/059671A2 proposes two engines having maximum powers that are not equal.
Document EP 0 091 744 appears to describe an aircraft having three identical engines. According to its description, the three engines can operate simultaneously. Nevertheless, the third engine may be shutdown in cruising flight. When all three engines are in use, a failure of the third engine is compensated by an increase in power from the main engines.
Documents EP 2 735 508, EP 1 978 214, EP 2 636 596, U.S. Pat. No. 5,403,155, and U.S. Pat. No. 3,869,862 are also known.